Magnetism found in Earth's mantle

New evidence come to challenge the current belief that the Earth's mantle is regarded as non-magnetic domain.

The study was recently published in Nature Journal and demonstrates that hematite (Fe₂O₃) , a common iron oxide, can preserve its magnetic properties even when being inside Earth's mantle. This phenomenon may occur in cold tectonic plates that are being subducted (slabs) and are mostly found in western Pacific Ocean.

The study may change the perceptive that magnetic observations are evaluated and could explain magnetic paradoxes that occur in Earth and in other planets. For example, Mars has no magnetic field deriving from its core but magnetism can be detected due to its mantle.

Background

The fact that the earth's mantle is believed to be “magnetically dead” is explained as following: The core of the planet consists of liquid iron alloy that causes electric flows. The rocks that appear in the crust of the Earth trigger the magnetic signal. However, it was believed that the magnetic properties of those rocks are eradicated in deeper regions, such as the Earth's mantle, due to the high pressure and temperature.

Experimental investigation

Scientists investigated the magnetic properties of iron oxides which are no longer magnetic after a critical temperature by conducting laboratory experiments. Iron oxides appear in the transition zone between the upper and the lower mantle of the Earth at a depth of 410-660 kilometers. For the first time, researchers managed to measure their magnetic properties in such conditions. In particular, they heated micro-sized samples of iron oxide that were squeezed together between two diamonds and managed to reach pressures of 90 GPa and temperatures of more than 1,000 °C.

Results

The results showed that iron oxides remain magnetic to a temperature of 925 °C. Therefore, their magnetic properties are preserved even in the Earth's mantle. “As a result, we are able to demonstrate that the Earth’s mantle is not nearly as magnetically ‘dead’ as has so far been assumed. These findings might justify other conclusions relating to the Earth’s entire magnetic field,” Prof. Carmen Sanchez-Valle at the Institute of Mineralogy at Münster University, stated.

The geomagnetic field of the Earth is constantly changing and therefore its poles switch positions but the manner that the overturn occurs is not understood yet. During the last years, scientists have reported significant acceleration in Earth’s magnetic poles. Now, researchers believe that some observations that are connected to the overturn may derive from magnetic sources inside the Earth’s mantle. “What we now know – that there are magnetically ordered materials down there in the Earth’s mantle – should be taken into account in any future analysis of the Earth’s magnetic field and of the movement of the poles,” Leonid Dubrovinsky co-author of the study and Professor at the Bavarian Research Institute of Experimental Geochemistry and Geophysics at Bayreuth University, stated.

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